Name: harvesting solar energy by means of charge-separating nanocrystals and their solids
Uploaded: 2012-08-23T12:03:00
Description: Watch this Scientific Journal Video about Harvesting Solar Energy by Means of Charge-Separating Nanocrystals and Their Solids at JoVE.com

We would like to acknowledge Dr. Felix Castellano (BGSU) and N. R. Neal for advice and valuable discussions. We gratefully acknowledge OBOR "Material Networks" program and Bowling Green State University for financial support. This work was partly supported by the NSF under Award CHE - 1112227.

1. Synthesis of ZnSe Core Nanocrystals8
<ol>
 <li>Place 7.0 g ODA and a magnetic stir bar into a three neck flask.</li>
 <li>In a separate flask, combine 0.063 g Se and 2.4 ml TOP and add a magnetic stir bar. The mixture of TOP and selenium should be degassed under vacuum for 30 min.</li>
 <li>Degas ODA for 90 min at 120 &deg;C, then put under Ar flow with a wide glass exhaust.</li>
 <li>Heat ODA to 300 &deg;C and, and inject Se mixture. Let temperature return to 300 &deg;C.</li>
 <li>Inject 1.0 ml of Et<sub...

<ol>
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This study demonstrates how composite architectures of inorganic nanocrystals can be employed to achieve a spatial separation of photoinduced charges. In particular, these composites allow fine-tuning of the distribution of charges across the two domains, which are then available to perform either photocatalytic or photovoltaic function. For instance, efficient photocatalysts can be made if donor and acceptor nanocrystal domains are built into a single nanoparticle. The energetics of such a system is shown in <st...

<table border="1">
 <tbody>
 <tr>
 <td>Name of the reagent</td>
 <td>Company</td>
 <td>Catalogue number</td>
 <td>Comments (optional)</td>
 </tr>
 <tr>
 <td>octadecylamine (ODA), 90%</td>
 <td>Fisher</td>
 <td>AC12932-0050 </td>
 <td></td>
 </tr>
 <tr>
 <td>selenium (Se), 200 mesh</td>
 <td>Acros</td>
 <td>AC19807-2500 </td>
 <td></td>
 </tr>
 <tr>
 <td>tri-n-octylphosphine (TOP), 97% </td>
 <td>Strem</td>
 <td>15-6655 </td>
 <td>Air Sensitive</td>
 </tr>
 <tr>
 <td>diethyl zinc (Et2Zn), 10% by wt.</td>
 <td>Aldrich</td>
 <td>22080 </td>
 <td>Air Sensitive, Light Sensitive</td>
 </tr>
 <tr>
 <td>methanol, 99.8%, anhydrous</td>
 <td>Aldrich</td>
 <td>179337</td>
 <td></td>
 </tr>
 <tr>
 <td>toluene, 99.8%, anhydrous</td>
 <td>Aldrich</td>
 <td>244511</td>
 <td></td>
 </tr>
 <tr>
 <td>tri-n-octylphosphine oxide (TOPO), 99% </td>
 <td>Aldrich</td>
 <td>223301 </td>
 <td></td>
 </tr>
 <tr>
 <td>n-octadecylphosphonic acid (ODPA), 98% </td>
 <td>PCI Synthesis</td>
 <td>104224</td>
 <td></td>
 </tr>
 <tr>
 <td>hexylphosphonic acid (HPA), 98%</td>
 <td>PCI Synthesis</td>
 <td>4721-24-8</td>
 <td></td>
 </tr>
 <tr>
 <td>cadmium oxide (CdO), 99.99%</td>
 <td>Aldrich</td>
 <td>202894</td>
 <td></td>
 </tr>
 <tr>
 <td>sulfur (S), 99.999%</td>
 <td>Acros</td>
 <td>AC19993-0500 </td>
 <td>Strong odor</td>
 </tr>
 <tr>
 <td>11-mercaptoundecanoic acid (MUA), 95%</td>
 <td>Aldrich</td>
 <td>450561</td>
 <td></td>
 </tr>
 <tr>
 <td>potassium hydroxide (KOH) </td>
 <td>Acros</td>
 <td>AC13406-0010 </td>
 <td></td>
 </tr>
 <tr>
 <td>chloroform</td>
 <td>VWR</td>
 <td>EM-CX1059-1 </td>
 <td></td>
 </tr>
 <tr>
 <td>lead oxide (PbO), 99.999%</td>
 <td>Aldrich</td>
 <td>32306-1KG </td>
 <td></td>
 </tr>
 <tr>
 <td>1-octadecene (ODE), 90%</td>
 <td>Aldrich</td>
 <td>O806-25ML</td>
 <td></td>
 </tr>
 <tr>
 <td>oleic acid (OA), 90%</td>
 <td>Aldrich</td>
 <td>O1008-1G</td>
 <td></td>
 </tr>
 <tr>
 <td>bis(trimethylsilyl) sulfide (TMS), synthetic grade</td>
 <td>Aldrich</td>
 <td>283134-25G</td>
 <td>Air sensitive, strong odor, highly reactive</td>
 </tr>
 <tr>
 <td>acetone</td>
 <td>EMD Chemicals</td>
 <td>AX0118-2</td>
 <td></td>
 </tr>
 <tr>
 <td>cadmium acetate</td>
 <td>Acros</td>
 <td>AC31713-5000 </td>
 <td></td>
 </tr>
 <tr>
 <td>sodium sulfide nonahydrate (Na2S&bull;9H2O), 98%</td>
 <td>Alfa Aesar</td>
 <td>CB1100945 </td>
 <td>Light sensitive</td>
 </tr>
 <tr>
 <td>hexadecyltrimethyl ammonium bromide (CTAB), 99%</td>
 <td>Sigma</td>
 <td>H6269-100G</td>
 <td></td>
 </tr>
 <tr>
 <td>oleylamine, 70%</td>
 <td>Aldrich</td>
 <td>O7805-5G</td>
 <td></td>
 </tr>
 <tr>
 <td>diphenyl ether</td>
 <td>Alpha Aesar</td>
 <td>101-84-8</td>
 <td></td>
 </tr>
 <tr>
 <td>1,2-hexadecanediol</td>
 <td>TCI</td>
 <td>6920-24-7</td>
 <td></td>
 </tr>
 <tr>
 <td>Pt (II) acetylacetonate, 97%</td>
 <td>Aldrich</td>
 <td>282782-5G</td>
 <td></td>
 </tr>
 <tr>
 <td>isopropanol, 99.8%, anhydrous</td>
 <td>Acros</td>
 <td>AC32696-0025 </td>
 <td></td>
 </tr>
 <tr>
 <td>titanium tetrachloride (TiCl4), 99.9%</td>
 <td>Aldrich</td>
 <td>697079-25G</td>
 <td>Extremely air sensitive</td>
 </tr>
 <tr>
 <td>titanium dioxide, DSL 90T</td>
 <td>DyeSol</td>
 <td>DSL 90T</td>
 <td></td>
 </tr>
 <tr>
 <td>terpineol</td>
 <td>MP Biomedical</td>
 <td>98-55-5</td>
 <td></td>
 </tr>
 <tr>
 <td>3-mercaptopropionic acid (MPA), 99%</td>
 <td>Alfa Aesar</td>
 <td>A10435</td>
 <td>Strong odor</td>
 </tr>
 <tr>
 <td>octane, anhydrous, 99%</td>
 <td>Aldrich</td>
 <td>412236</td>
 <td></td>
 </tr>
 </tbody>
</table>

harvesting solar energy by means of charge-separating nanocrystals and their solids

Conjoining different semiconductor materials in a single nano-composite provides synthetic means for the development of novel optoelectronic materials offering a superior control over the spatial distribution of charge carriers across material interfaces. As this study demonstrates, a combination of donor-acceptor nanocrystal (NC) domains in a single nanoparticle can lead to the realization of efficient photocatalytic1-5 materials, while a layered assembly of donor- and acceptor-like nanocrystals films gives rise to photovoltaic materials.
Initially the paper focuses on the synthesis of composite inorganic nanocrystals, comprising linearly stacked ZnSe, CdS, and Pt domains, which jointly promote photoinduced charge separation. These structures are used in aqueous solutions for the photocatalysis of water under solar radiation, resulting in the production of H2 gas. To enhance the photoinduced separation of charges, a nanorod morphology with a linear gradient originating from an intrinsic electric field is used5. The inter-domain energetics are then optimized to drive photogenerated electrons toward the Pt catalytic site while expelling the holes to the surface of ZnSe domains for sacrificial regeneration (via methanol). Here we show that the only efficient way to produce hydrogen is to use electron-donating ligands to passivate the surface states by tuning the energy level alignment at the semiconductor-ligand interface. Stable and efficient reduction of water is allowed by these ligands due to the fact that they fill vacancies in the valence band of the semiconductor domain, preventing energetic holes from degrading it. Specifically, we show that the energy of the hole is transferred to the ligand moiety, leaving the semiconductor domain functional. This enables us to return the entire nanocrystal-ligand system to a functional state, when the ligands are degraded, by simply adding fresh ligands to the system4.
To promote a photovoltaic charge separation, we use a composite two-layer solid of PbS and TiO2 films. In this configuration, photoinduced electrons are injected into TiO2 and are subsequently picked up by an FTO electrode, while holes are channeled to a Au electrode via PbS layer6. To develop the latter we introduce a Semiconductor Matrix Encapsulated Nanocrystal Arrays (SMENA) strategy, which allows bonding PbS NCs into the surrounding matrix of CdS semiconductor. As a result, fabricated solids exhibit excellent thermal stability, attributed to the heteroepitaxial structure of nanocrystal-matrix interfaces, and show compelling light-harvesting performance in prototype solar cells7.

Watch this Scientific Journal Video about Harvesting Solar Energy by Means of Charge-Separating Nanocrystals and Their Solids at JoVE.com

Harvesting Solar Energy by Means of Charge-Separating Nanocrystals and Their Solids

A general strategy for the development of charge-separating semiconductor nanocrystal composites deployable for solar energy production is presented. We show that assembly of donor-acceptor nanocrystal domains in a single nanoparticle geometry gives rise to a photocatalytic function, while bulk-heterojunctions of donor-acceptor nanocrystal films can be used for photovoltaic energy conversion.

Conjoining different semiconductor materials in a single nano-composite provides synthetic means for the development of novel optoelectronic materials ...

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